Seismic exploration involves surveying subterranean geological media for hydrocarbon deposits. A survey typically involves deploying seismic sources and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological medium creating pressure changes and vibrations. Variations in physical properties of the geological medium give rise to changes in certain properties of the seismic waves, such as their direction of propagation and other properties.
Portions of the seismic waves reach the seismic sensors. Some seismic sensors are sensitive to pressure changes (e.g., hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy one type of sensor or both. In response to the detected seismic waves, the sensors generate corresponding electrical signals, known as traces, and record them in storage media as seismic data. Seismic data will include a plurality of “shots” (individual instances of the seismic source being activated), each of which are associated with a plurality of traces recorded at the plurality of sensors.
Seismic data is processed to create seismic images that can be interpreted to identify subsurface geologic features including hydrocarbon deposits. In some cases, particularly in areas of complex geology, faults may cut through suspected hydrocarbon reservoirs. Depending on their geometry, lithologic juxtapositions, and stress states, faults can prohibit, impede, or enhance the movement of oil, gas, and water through hydrocarbon reservoirs. Accurate prediction of this behavior is important for the efficient and effective exploration and exploitation of oil and gas accumulations. Traditional fault characterization workflows are based on analysis of the juxtaposition of geologic sequences whose positions are interpreted from seismic reflection, well, and surface geologic data. Traditionally, faults are analyzed in 3D seismic images using a combination of visual inspection of lateral variations in horizon reflection character and multi-trace attributes. Both techniques identify and characterize faults based on differences in amplitude and/or phase of the adjacent horizon reflections. In these analyses, the quantitative character of the fault surface reflection (or lack thereof) is neither measured nor used in characterization of the fault's effect on fluid flow.
A few studies report using fault reflection signal for pore-pressure correlation (Haney et al. 2005, 2007). They describe use of slant stacks to enhance fault reflection signal, extract maximum amplitude within a window, map it onto fault surface, and qualitatively correlate the maximum amplitude with pressure difference across fault. This analysis lacks 1) capturing the full response of the fault surface and its surroundings and 2) the ability to quantitatively correlate reflection signal from the fault to other geological information. Botter et al. (2014, 2016) used discrete element and pre-stack depth migration modeling approach to understand seismic response of faults. This is an attempt to obtain insight of the fault from modeling and potentially tie modeled seismic to field observation. However, this approach does not provide quantitative information. The convolutional seismic modeling rather than realistic image modeling (with realistic complexity) simplifies overburden way too much. Additionally, the parameters used in the model may not be accurate because the forward modeling may or may not match field observations and multiple parameter combinations may produce similar outcomes.
The ability to define the location of rock and fluid property changes in the subsurface, including those across faults, is crucial to our ability to make the most appropriate choices for purchasing materials, operating safely, and successfully completing projects. Project cost is dependent upon accurate prediction of the position of physical boundaries within the Earth. Decisions include, but are not limited to, budgetary planning, obtaining mineral and lease rights, signing well commitments, permitting rig locations, designing well paths and drilling strategy, preventing subsurface integrity issues by planning proper casing and cementation strategies, and selecting and purchasing appropriate completion and production equipment.
There exists a need for predicting fault seal in order to reduce risk in drilling into potential hydrocarbon reservoirs.